The general trend now is to reduce the quantity of benzenes and olefins (unsaturated compounds) in gasolines, because of their known toxicity.
Benzene has carcinogenic properties and thus the possibility of it polluting the air must be limited as far as possible, in particular by practically excluding it from automobile fuels. In the United States, reformulated fuels must not contain more than 1% by volume of benzene; in Europe, it has been recommended that a gradual decrease towards that value be made.
The benzene content of a gasoline is very largely dependent on that of the reformate component in that gasoline. The reformate results from catalytic treatment of naphtha intended to produce aromatic hydrocarbons, principally comprising 6 to 9 carbon atoms per molecule and the octane number of which is very high endowing the gasoline with antiknock properties.
Because of the toxicity described above, the amount of benzene in the reformate must be reduced to acceptable levels.
The benzene in a reformate can be hydrogenated to cyclohexane. Since it is impossible to selectively hydrogenate benzene in a mixture of hydrocarbons also containing toluene and xylenes, that mixture must first be fractionated to isolate a cut containing only benzene, which can then be hydrogenated.
International patent application WO 95/1 5934 describes a reactive distillation which aims to selectively hydrogenate diolefins and C2-C5 acetylenic compounds. The distillate can be separately recovered from the light compounds. The catalytic hydrogenation zone is completely internal to the distillation column, which means that the hydrogen cannot dissolve properly in the feed and the pressure cannot be increased.
A process has been described in which the catalytic benzene hydrogenation zone is internal to the distillation column has been described which separates benzene from other aromatic compounds (Benzene Reduction--Kerry Rock and Gary Gildert CDTECH--1994 Conference on Clean Air Act Implementation and Reformulated Gasoline--October 94), which cuts the cost of the apparatus. It appears that the pressure drop across the catalytic bed(s) in that process means that an intimate mixture between the liquid phase and the gaseous stream containing the hydrogen cannot be obtained. In that type of technology where the reaction and distillation proceed simultaneously in the same physical space, the liquid phase descends through every catalytic bed in the reaction zone in a trickle flow, and thus in threads of liquid. The gaseous fraction containing the fraction of vaporised feed and the gas stream containing hydrogen rise through the catalytic bed in columns of gas. In that arrangement, the entropy of the system is high and the pressure drop across the catalytic bed(s) is low. As a result, operating that type of technique cannot easily promote dissolution of hydrogen in the liquid phase comprising the unsaturated compound(s).
European published patent application EP-A-0 781 830 assigned to Institut Francais du Petrol describes a process for hydrogenating benzene using a distillation column associated with a reaction zone which is at least partially external. The feed for the reaction zone is withdrawn from the distillation zone then the effluent from the reaction zone is re-introduced into the distillation zone. The hydrogenation reaction can also take place in the distillation zone. This process does not envisage a circulating reflux in the column with the result that no heat is extracted from the reaction zone.